Читать книгу Earth Materials - John O'Brien - Страница 47

Atoms in minerals, rocks, and other Earth materials are held together by forces or mechanisms called chemical bonds. The nature of these bonds strongly influences the properties and behavior of these materials. The nature of the bonds is, in turn, strongly influenced by the electron configuration of the elements that combine to produce the mineral, rock or other material.

Five principle bond types and many hybrids occur in minerals. The three most common bond types are (1) ionic, (2) covalent, and (3) metallic. They can be modeled based on the behavior of valence electrons in the outer quantum levels of atoms. During bonding, valence electrons display varying tendencies to change position based on their periodic properties. In discussing chemical bonds, it is useful to divide elements into those that are metallic and those that are nonmetallic.

Ionic bonds involve the linking together of metallic and nonmetallic elements, covalent bonds involve the linking of two nonmetallic elements, and metallic bonds involve the linking of two metallic elements. Hybrids between these bond types are common. Minerals with such hybrid or transitional bonds commonly possess combinations of features characteristic of each bond type. Other bond types include van der Waals and hydrogen bonds. Chemical bonding is a very complicated process; the models used below are simplifications designed to make this complex process easier to understand to a reasonable degree.

Another useful concept for understanding chemical bonds, developed originally by Linus Pauling (1929), is the concept of electronegativity (see Table 2.3). Electronegativity (En) is an empirical measure that expresses the tendency of an element to attract electrons when atoms bond. Highly electronegative elements (En > 3.0) have a strong tendency to attract electrons and become anions during bonding. Many column 16 (group VIA) and column 17 (group VIIA) elements are highly electronegative, requiring capture of two or one electrons, respectively, to achieve a stable electron configuration. Elements with low electronegativity (En < 1.5) are electropositive, metallic elements with a tendency to give up electrons to more electronegative elements during bonding to become positively charged cations. Highly electropositive elements include column 1 (group IA) and column 2 (group IIA) elements that tend to release one or two electrons, respectively, to achieve a stable electron configuration. Electronegativity is a very helpful concept in discussions of how atoms bond to produce larger molecules and minerals.